Solid polymer electrolytes, SPE's, are ionic polymers having very high ion conductivity. As electrolytes they are useful in electrochemical systems, primarily in batteries and fuel cells. The polymeric nature of SPE's makes them much easier to handle than liquid electrolytes. The physical construction of the electrochemical cell is greatly simplified with the use of SPE's since elaborate seals and containment systems are not needed to contain corrosive liquid electrolytes. The use of SPE's in fuel cells and batteries is well established in the art.
The use of solid polymer electrolytes can greatly simplify cell design. Liquid electrolytes must be confined with a separator and contained with seals. Liquid electrolytes are highly corrosive and more readily contaminated than solid electrolytes. Fuel and oxygen will diffuse across liquid electrolytes more readily, lowering efficiency. SPE's avoid these problems and can be made thinner, thereby lowering cell resistance. Handling of SPE's is much easier than handling liquid systems, and cell construction can be simpler.
Fuel cells with SPE's promise greater energy density than liquid electrolyte cells because of low overall weight, primarily due to simpler construction and thinner cells. The first fuel cells flown in the U.S. space program used sulfonated polystyrene SPE's, and SPE cells are still a choice for space missions. (Alkaline cells are also used.)
An SPE should have the following properties: (1) high ionic conductivity, (2) zero electronic conductivity, (3) very low permeability to gases, (4) chemical stability at the operating temperature, (5) mechanical strength, (6) low sensitivity to humidity, and (7) compatibility with catalyst.
The first of these is by far the most difficult to obtain. Current SPE's must be operated at temperatures and pressures where water is a liquid; otherwise, the membrane dehydrates, and proton conductivity is drastically reduced. Although byproduct water must be removed, care must be taken not to dry out the SPE. Water management is a major difficulty of currently available SPE's. Fuel and air streams must be pre-humidified, and temperature strictly limited to avoid dehydration. These extra control systems add significant weight and cost.
It would be highly advantageous to operate SPE fuel cells above the boiling point of water. This would greatly simplify water balance. Temperatures lower than about 80.degree. C. require active cooling with concomitant weight and cost. The key lies in development of an SPE with high proton conductivity in the absence of condensed water. The need for hydration with sulfonated polyfluorocarbon SPE's is a result of the relatively low concentration of sulfonate groups and the hydrophobic nature of the fluorocarbon backbone. The structure of perfluorosulfonated polymer membranes is such that the sulfonate groups tend to concentrate in a water rich phase, which forms a network permeating the hydrophobic fluorocarbon regions. When dehydrated, the sulfonate groups become isolated, and proton migration between groups is difficult.
There is therefore a need for SPE's which maintain high proton conductivity above the boiling point of water and in the absence of liquid water.